Cutting units are used chiefly in the office sector to trim sheets of paper or film, either individually or as a stack of sheet material. A feature common to such cutting units is that they have a horizontal support plate for placement of the sheet material, and that a cutting device is provided on at least one side of the support plate. Rotary cutting devices are suitable for producing particularly smooth cut edges.
A rotary cutting device comprises a guide bar that extends parallel to one side of the support plate and also parallel to the surface of the support plate. A knife carriage is guided on the guide bar, displaceably along the guide bar. The knife carriage comprises a rotary knife that is mounted, freely rotatably about an axis extending horizontally and transversely to the longitudinal axis of the guide bar, on the knife carriage. Usually the knife carriage itself, or the part of the knife carriage on which the rotary knife is mounted, can be lowered vertically from above onto the support plate by exerting pressure.
For a cutting operation, the sheet material is placed onto the support plate and is slid through beneath the guide bar until the portion to be cut off protrudes on the other side of the guide bar. Vertical pressure is then exerted on the knife carriage so that the rotary knife is lowered toward the sheet material. Cutting of the sheet material is then accomplished by displacing the knife carriage along the guide bar. The rotary knife performs a rotary motion in this context, and thereby cuts the sheet material.
To eliminate the need to hold the sheet material with the other hand during the cutting operation, a variety of solutions have been found with which the sheet material can be clamped against the support plate. In the rotary cutting units according to U.S. Pat. Nos. 5,671,647 A and 5,365,820 A, the guide bars rest on a spring device having end-located spring members that are braced on the support plate. When pressure toward the support plate is exerted on the knife carriage during the cutting operation, the guide bar is pushed downward into a clamping position and thereby grips the sheet material between the guide bar and support plate.
The rotary cutting unit according to U.S. Pat. No. 6,966,247 B2 is similar in its action. Here the guide bar is attached at its ends rigidly to the support plate, but located beneath the guide bar is a vertically movable clamping bar that, in the initial position, is clamped via tension springs against the underside of the guide bar. The knife carriage is divided into a guidance part and a knife part, the knife part having the rotary knife being vertically displaceable with respect to the horizontally extending guide bar. When vertical pressure is exerted on the knife carriage, its movable part is lowered toward the support plate and the clamping bar is carried along. The latter then abuts against the sheet material to be trimmed, and immobilizes it.
A disadvantage of the rotary cutting units described above is that clamping of the sheet material is dependent on the action on the knife carriage, i.e. clamping is not accomplished until the cutting operation begins, and moreover is locally concentrated on the position of the knife carriage. Prior to the cutting operation, shifting of the sheet material can therefore still occur.
To avoid this, rotary cutting units have been developed that effect a clamping of the sheet material independently of the actuation of the knife carriage (cf. U.S. Pat. Nos. 5,069,097 A and 5,287,783 A). In the context of their rotary cutting devices, a clamping bar is likewise arranged beneath the guide bar and is suspended from the guide bar by springs. The guide bar itself is vertically movable between a raised initial position and a final position. Before a cutting operation and after positioning of the sheet material, the guide bar is lowered from the initial position toward the support plate and the sheet material until it has reached the final position. The clamping bar is thereby set onto the sheet material and clamps it against the support plate. The clamping pressure is applied by the springs between the guide bar and clamping bar. In the final position, the guide bar is held at its ends, via guidance devices and immobilization devices, in a position that is always the same and is therefore independent of the thickness of the sheet stack.
In the embodiment according to U.S. Pat. No. 5,069,097 A, the guidance device is embodied on one side of the guide bar as a pivot joint by means of which the guide bar can be pivoted upward in lever fashion. In the initial position, the guide bar on the other side is located outside the guidance device and latches therewith once the final position is assumed. In the embodiment according to U.S. Pat. No. 5,287,783 A, actuation devices are provided on both sides; with said devices, the guide bar can be transported out of an initial position parallel to the support plate into the final position, where it is immobilized.
The above-described rotary cutting units secure the sheet material in position independently of the cutting operation, but are of relatively complex construction and therefore expensive and prone to malfunction.
With the rotary cutting unit according to US 2005/0120850 A1, a solution has been found that eliminates the aforesaid disadvantages. With this rotary cutting device, once again the guide bar itself serves as a clamping device to clamp the sheet material against the support plate. For this purpose, the guide bar is held at both ends in guidance devices, in a first embodiment on one side via a pivot joint and on the other side in substantially vertically displaceable fashion, and in a second embodiment vertically displaceably on both sides. Wherever the guide bar is vertically movable, a respective actuation device is provided with which the guide bar can be brought out of the initial position, in which its lower-side clamping surface is at a distance from the support plate, into a clamping position in which the clamping surface rests with a preload on the support plate or, when sheet material has been slid under, on that material itself and clamps it against the support plate. An immobilization device in the form of a latching device ensures that the guide bar, once having assumed the clamping position, maintains it independently of the cutting operation. Only by releasing the latching device(s) is the guide bar again lifted away from the sheet material into the initial position, specifically by means of spring members of a spring device.
The above-described rotary cutting unit is notable for a simple design configuration. It is necessary, however, to accept that, because of the defined clamping position of a guide bar, only a small number of sheets can be trimmed simultaneously.